He's about 1 year old, a miniature schnauzer, salt and pepper coloring. I noticed a marble sized lump on his stomach while petting him. There's no irritation around it. We are taking him to the vet very soon, but I'm worried! Until then do you think it's cancer or a tumor or something?

It is most likely a hernia. If it's right in the midline, below his rib cage, it is probably an umbilical hernia. Small ones can come and go and are usually repaired when the dog is anesthetized or neutering/spaying. If a loop of bowel becomes trapped in a hernia, it is life-threatening. There is a genetic susceptibility to hernias so dos which have them should not be bred.